Jordy van Sambeeck

Patellar height measurements: Insall-Salvati ratio is most reliable method 2 31 Introduction Patella alta is a well-known anatomic risk factor for recurrent patellar instability which may contribute to patellofemoral pain [12,15,34]. In patients with patellar instability or patellofemoral pain a measurement of patellar height should be included in the workup for treatment because surgical correction of patellar height might be indicated. To measure patellar height several measurement methods and imaging modalities are in use. Measurement methods include the Insall- Salvati ratio (IS) [21], Blackburne-Peel ratio (BP) [7], Caton-Deschamps ratio (CD) [10], modified Insall-Salvati ratio (MIS) [19] and patellotrochlear index (PTI) [5] (Fig. 1). Unfortunately, there is still no consensus in literature on measurement method or cutoff value as shown by Biedert et al [6] in their recent review of patellar height measurement methods. Widely used imaging modalities are conventional radiography (CR), computed tomography (CT) and magnetic resonance imaging (MRI). Recently however Giovagnorio et al [17] proposed ultrasound as a good imagingmodality tomeasure patellar height, reducing the need for other imaging techniques. IS, BP, CD and MIS were originally designed for measurement on CR. These methods could also be applied to CT or MRI images, in which case different normal values might be used. Lee et al. [23] present normal values for IS and BP for different imaging modalities. These have not been described for MIS and CD to our knowledge. Apart from the confined amount of literature describing normal values for CT and MRI, there is also a lack of descriptions of standardized measurements of patellar height on these imaging modalities. The measurement technique differs from measurement on CR, among other things because of the presence of slices on CT and MRI. Shabshin et al. [32] describe a method to standardize the choice for a slice to measure patellar length and patellar tendon length. It is not known if this is a reliable method between observers or if it is applicable to all measurement methods, and it is time consuming to apply this in daily practice. Barnett et al. [3] describe good inter- and intra-observer reliability for IS, BP, CD and PTI on MRI. However, their choice of slice on MRI is based on the PTI, which was specifically designed for measurement of patellar height onMRI using the femur as a reference point. This is in contrast to IS, MIS, CD and BP where the tibia is used as reference. Ali et al. [1] and Barnett et al. [3] describe that the PTI does not correlate well with other patellar height measurements. Therefore, this might not be a good way to choose a slice on which to perform other patellar height measurements. The aim of this study was to determine the intra-and interrater reliability for different patellar height measurement methods (IS, BP, CD, MIS originally designed for CR) on CR, CT and MRI. This includes the intra-and interrater reliability for the PTI on MRI. It was hypothesized that there might be significant variability in measurement results between measurements and between imaging modalities.